ACQUISITIONS (Pro Forma) (Details) (USD $)
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]      
Pro forma total revenues $ 254,957,000 $ 203,093,000 $ 167,671,000
Pro forma net income 40,661,000 32,167,000 54,405,000
Limited partners, Common
     
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]      
Pro forma EPU, basic and diluted $ 0.74 $ 0.29  
Subordinated units
     
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]      
Pro forma EPU, basic and diluted $ 0.74 $ 0.29  
Acquisition-related Costs
     
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]      
Pro forma effect of nonrecurring transaction costs incurred on net income 2,500,000    
Bison Midstream and Mountaineer Midstream
     
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]      
Incremental borrowings on revolving credit facility 310,000,000    
Pro forma total revenues 60,323,000 0  
Pro forma net income (457,000) 0  
Grand River Gathering, LLC
     
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]      
Pro forma total revenues     12,824,000
Pro forma net income     $ 2,660,000